Reality and confusion in the recognition of post-depositional alterations and use-wear: an experimental approach on basalt tools

Abstract

While many experimental studies have been done on siliceous and metamorphic rocks for both use-wear and post depositional surface modification (PDSM) events, little is known about such experiments on volcanic materials (other than obsidian), and on basalt in particular. Here we present the preliminary results of several experiments related to: a) evidence for basalt use (e.g., butchery and fresh hide scraping) and the subsequent characteristic use-wear patterns that can be seen; and b) post-depositional surface modifications caused by bear (Ursidae) trampling on experimental basalt flakes and subsequent use of these flakes for a tumbling experiment in a special tumbling machine. The results of these experiments were compared to better understand some surface modifications noted on the Middle to Upper Pleistocene lithic assemblages of the Azokh Cave site (Nagorno Karabagh, Lesser Caucasus). Although some aspects of both events (use-wear and PDSM) remain to be studied in depth, the experiments have improved our understanding of the effects of use-wear and post-depositional trampling and tumbling on basalt lithic artefacts. In particular, it has allowed us to recognise mechanical alterations (e.g., cracks, striations, fractures, edge damage) caused by trampling and tumbling and to note differences between these modifications and those caused by use. In particular, the experiments have shown that macroscopic modifications are rarely diagnostic, especially those observed after use. Microscopic wear features such as edge rounding, polish, abrasion and striations were the most evident types of alteration on basalt flakes, although occurring at different times and from different types of use. Distribution and orientation of alterations may be key in distinguishing use-wear from post-depositional alterations on basalt artefacts.

Keywords

• experimental case-studies
• basalt
• use-wear patterns
• post-depositional surface modifications (pdsm)